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As the islamqa article mentions: Dying hair pure black is Makruh(disliked) and not haram(forbidden) according to the Hanafis, Malikis and Hanbalis. The Shafis differ about it, with Nawawi considering it haram and Gazali considering it makruh.

The part about it being allowed in war is explained here. Its permissible as a stratagem of war, in order to give the impression that the soldiers of the Muslim army are all in their prime youth. Its similar to the permissibly of silk in a warrior's armor [Hidayah p597], [Nawawi's Minhaj al Talibin p64].